Tens of thousands of wind turbines have been installed over the past decade, almost all using a similar drive system incorporating a gearbox as a speed increaser, positioned between the turbine blades and a generator. Gearboxes are typically designed with the intent and desire to ensure that the bearings and gears are suitably aligned to take their intended design loads. Those design loads are typically focused on the forward operating rotational direction. However, it has been found that high reverse torques can affect the life of the bearings and gears of the gearbox. During torque reversals, the gears and bearings become misaligned, causing highly concentrated loading on the contact surfaces. Even a moderate reverse torque spike can damage the misaligned bearings and gears. There are various operating conditions that induce high torsional vibrations in the drive train and gearbox, some of which can cause severe torque in the reverse drive direction. These conditions may arise (a) upon start up when the electric contactor engages the wind turbine generator to the grid; (b) during emergency braking; (c) during normal braking when the caliper brake engages; (d) during grid disconnect events; and (e) during any of various electrical faults and control malfunctions.
While gearboxes for wind turbines are typically designed for a 20 year bearing and gear life, it has been found that the average life of gearboxes in wind turbine designs is on the order of 7-11 years. The cost of gearbox replacement is extremely high, not only in direct costs, but in downtime, as well. Indeed, it is believed that premature gearbox failure in many wind turbine designs has been largely a consequence of reverse torque load, for which no effective protection has been provided. Many wind turbines have traditional friction torque limiting couplings, typically set at 150-200% of the rated torque of the wind turbine. These do not provide adequate protection for misaligned bearings and gears loaded in reverse. The present invention contemplates that an asymmetric torque limiter coupling system, with a high torque setting in the standard forward direction and a low torque setting in the reverse direction could significantly improve gearbox life.